[Bug ipa/91089] New: IPA-cp does setup proper cost model for switch default case in function versioning

[fxue at os dot amperecomputing.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91089

            Bug ID: 91089
           Summary: IPA-cp does setup proper cost model for switch default
                    case in function versioning
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fxue at os dot amperecomputing.com
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

IPA-cp always adds execution cost of switch default case into cost evaluation
for other switch case, which might miss opportunities of function versioning
when only certain case is proved to be executed after constant propagation.

The following is an example, compiled with "-O3 -fno-inline".

It is reasonable to make a function versioning for callee when "v=1", but
actually it does not happen. And if we replace "default" with "case = 4",
it clones the function as we expect.

In some situations, we can deduce a relative small execution predicate
for default case. If all switch cases are in a continuous value range, we
can use compare to range low and high bound to mark default switch case.
Additionally, range analysis can also give us more information to simplify
the predicate.

int foo();

int callee(int v)
{
  int i = 0;

  switch (v)
   {
     case 1:
       i = 3;
       break;
     case 2:
       i = 9;
       break;

     default: 
       foo();
       foo();
       foo();
       foo();
       foo();
       foo();
       foo();
       foo();
       foo();
  }
  return i;
}

int caller()
{
  return callee (1);
}